1. Field of the Invention
The present invention relates generally to a metal detection system and method, more particularly to a Wide Area Metal Detection (WAMD) system and method for security screening crowds of people.
2. Description of the Related Art
Metal detectors are used for security purposes in a number of locations, such as airports, sporting events, concerts, amusement parks, federal buildings, banks, schools and other high-security installations. Commonly, there are two types of metal detectors in use in such installations. The first type includes a transmitting coil located on one side of the detector and a receiving coil located on the opposite side of the detector. Typically, a magnetic field is generated on one side of the detector by the transmitting coil, and the generated field is received on the other side of the detector by the receiving coil. As long as the magnetic field received by the receiving coil is within the predetermined parameters programmed into the detector, an alarm is not actuated. However, the passage or presence of a metallic object through or in the magnetic field causes a disturbance in the field received by the receiving coil. If this disturbance causes the magnetic field to fall outside of the predetermined parameters, the alarm associated with the detector is actuated.
Another type of ferromagnetic metal detector is disclosed in U.S. Pat. No. 3,971,983 to Jaquet. This type of detector employs a number of gradiometers positioned on both sides of a walk-through portal. While this device does not actively generate a magnetic field within the portal, the gradiometers monitor the magnetic field generated by the earth. Any disturbances in the earth's magnetic field, such as may be caused by the presence of a ferromagnetic object within the portal, are detected by the gradiometers, resulting in the activation of an alarm.
While these types of detector systems can be very accurate, typically, the operation of and hardware associated with the systems only allow one person to walk through the metal detector at a time. This creates a “choke point” and a line typically forms. For large sporting events, military checkpoints, border crossings and anywhere where large crowds gather and must be screened for metal weapons or contraband, the use of portable type metal detectors create problems.
Whitehead, et al. U.S. Pat. No. 2,560,834 and Keller U.S. Pat. No. 5,859,532 disclose apparatuses for detecting and locating a buried, metallic object that use a single transmitter loop (Whitehead; vertical magnetic field generator) or a series of transmitter loops (Keller) and a linear, one-dimensional, array of receiver coils (vertical magnetic field receivers). The apparatuses are then moved over the ground to locate buried metal objects. None of these apparatuses are placed under ground, nor do either of these patents disclose a means to track and verify the potential source of the person in a crowd carrying a metal object. In addition, an extensive, two-dimensional array of transmitter loops and receiver loops are needed to track with high confidence a person with metal in a crowd. Such an extensive array over a large area may be expensive and difficult to deploy and maintain.
Goldfine, et al. U.S. Pat. No. 6,144,206 discloses a magnetic transmitter configuration that is more appropriate for a large survey area for buried metal objects (e.g., land mines and unexploded ordnance). Goldfine employs a relatively complex spatial and temporal sampling scheme for the transmitter and receivers to aid in target localization. The Goldfine apparatus has the same problem as the Whitehead and Keller apparatuses for tracking a moving person with metal in a crowd. No provision is made for a combined metal detector and video tracking system.
Accordingly, a need exists for wide-area metal detection screening, which can be used to locate potential people in a crowd that should be investigated further. This pre-screening greatly reduces the need for everyone entering a building or an area to be “scanned” for potential weapons with a high sensitivity portal-type metal detector or a handheld metal detector.